1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to configurations of side edge regulating portions configured to regulate positions of side edges of sheets stored in a sheet feeding cassette detachably mounted to an apparatus main body.
2. Description of the Related Art
There is now widely used an image forming apparatus, such as a copier, printer, or facsimile, in which a sheet feeding apparatus is used to feed a sheet to an image forming portion, to thereby form an image. Further, in such a sheet feeding apparatus, in general, a sheet feeding cassette is detachably mounted to an apparatus main body, and sheets stored in the sheet feeding cassette are automatically fed to the image forming portion.
Examples of the sheet feeding cassette used in such a sheet feeding apparatus include one in which a lifting plate for stacking sheets thereon and pressing the sheets onto a sheet feeding roller is provided so as to freely rise and lower. In addition, in the sheet feeding cassette provided with such a lifting plate, a trailing edge regulating portion for regulating positions of upstream edges (hereinafter, referred to as trailing edges) in a sheet feeding direction of the sheets stacked and stored on the lifting plate is provided so as to be slidable in order that sheets having different sizes can be stored. Further, the sheet feeding cassette is provided with a pair of side edge regulating portions for regulating side edge positions of the sheet in a direction (hereinafter, referred to as width direction) orthogonal to the sheet feeding direction.
Further, the pair of side edge regulating portions regulates the side edges of the sheet, whereas the trailing edge regulating portion regulates the trailing edge of the sheet. Accordingly, a leading edge position of the sheet is always regulated to be located at a predetermined position. With this configuration, when the sheet feeding cassette is accommodated to the apparatus main body, it is possible to perform a stable sheet feeding operation regardless of a sheet size.
Examples of the sheet feeding cassette provided with such side edge regulating portions include one in which one of the side edge regulating portions is fixed, whereas the other of the side edge regulating portions is slidable in the width direction. Further, the fixed one of the side edge regulating portions (hereinafter, referred to as a stationary side edge regulating portion) is used as a reference surface in the width direction for the sheets. By feeding the sheet along the stationary side edge regulating portion, a predetermined printing accuracy is achieved.
By the way, in the sheet feeding cassette as described above, in a case where an attachment/detachment direction of the sheet feeding cassette is aligned parallel with the width direction, when the sheet feeding cassette is set, the sheets may be shifted due to an inertial force and separated from the stationary side edge regulating portion. Thus, conventionally, of the pair of side edge regulating portions, the slidable side edge regulating portion (hereinafter, referred to as a movable side edge regulating portion) is provided with a sheet pressing portion for pressing the sheets. Further, in a case where the sheets are shifted, the sheet pressing portion pushes back the shifted sheets to the stationary side edge regulating portion.
Here, in order to push back the thus shifted sheets to an original position, it is necessary to make a pressing force of the sheet pressing portion strong. However, in a case where the pressing force of the sheet pressing portion is set to such a magnitude as to be capable of pushing back the sheets when an amount of stack of the sheets is large, for example, at the time of full-level stack of the sheets, the sheets warp on the lifting plate at the time of low-level stack when the amount of stack of the sheets is small, and hence cannot keep appropriate postures. That is, in a case where the pressing force of the sheet pressing portion is set to such a magnitude as to be capable of pushing back the sheets to the original position at the time of full-level stack, the sheets warp on the lifting plate at the time of low-level stack, and hence cannot keep appropriate postures.
In this context, conventionally, there is proposed an invention which prevents occurrence of warpage of the sheets at the time of low-level stack while increasing the pressing force to the sheets at the time of full-level stack. For example, the following configuration is proposed. Specifically, the pressing force of the sheet pressing portion is actively varied using a link, and the pressing force is weakened under a state in which the amount of stack of the sheets is small. This technology is disclosed in Japanese Patent Application Laid-Open No. 2000-118730. However, with this configuration, the number of components increases, and hence cost increases. Thus, in order not to increase the number of components, the following configuration is proposed. Specifically, a relief surface is formed on the sheet pressing portion. Owing to the relief surface, at the time of low-level stack, the pressing force of the sheet pressing portion is reduced, or the sheets are prevented from contacting with the sheet pressing portion so that the sheet pressing portion does not press the sheets. This technology is disclosed in Japanese Patent Application Laid-Open No. 2000-219330.
Note that, FIGS. 8A and 8B are perspective views illustrating a sheet feeding cassette provided with a movable side edge regulating portion in which a relief surface is formed on a sheet pressing portion. FIG. 8A illustrates a state in which an amount of stack of sheets S is small, and FIG. 8B illustrates a state in which the amount of stack of the sheets S is full. Here, in this state, a lifting plate 713 is raised by an urging unit (not shown), and the sheets S on the lifting plate 713 are held in press-contact with a sheet feeding roller 708. Further, at this time, a movement in the width direction of the sheets S is regulated by a stationary side edge regulating portion 714 and a movable side edge regulating portion 715. Further, of sheet pressing portions 716 and 717 provided on the movable side edge regulating portion 715, the sheet pressing portion 716, which is located at the center portion in the sheet feeding direction indicated by an arrow, has a relief shape 716a formed for reducing the pressing force exerted on the sheets S at the time of low-level stack.
However, in the above-mentioned sheet feeding apparatus and image forming apparatus, for example, in a case where the relief surface is formed on the sheet pressing portion, as illustrated in FIG. 9A, a space 716b is formed below the relief shape 716a and between the sheets S and the sheet pressing portion 716 at the time of full-level stack of the sheets. Thus, when the sheet feeding cassette is inclined under a state in which the sheets S are set, or when impact is generated when the sheet feeding cassette is attached to the apparatus main body, as illustrated in FIG. 9B, of the sheets S, the sheets that are not held in contact with the sheet pressing portion 716 are shifted into the space 716b. When the lifting plate 713 is raised in this state, there is a risk in that the shifted sheets hamper the pressing force of the sheet pressing portion 716 exerted by a spring 901, or the shifted sheets S are fed in the shifted postures.
In recent years, the sheet feeding cassette is required to store a wide variety of sheets and to have higher stacking performance. However, with the above-mentioned conventional configuration, it is difficult to achieve sufficient feeding performance.